speculative execution does not handle cases where stddev > mean well
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Key: MAPREDUCE-2162
URL: https://issues.apache.org/jira/browse/MAPREDUCE-2162
Project: Hadoop Map/Reduce
Issue Type: Bug
Reporter: Joydeep Sen Sarma
the new speculation code only speculates tasks whose progress rate deviates from the mean
progress rate of a job by more than some multiple (typically 1.0) of stddev. stddev can be
larger than mean. which means that if we ever get into a situation where this condition holds
true - then a task with even 0 progress rate will not be speculated.
it's not clear that this condition is self-correcting. if a job has thousands of tasks - then
one laggard task, inspite of not being speculated for a long time, may not be able to fix
the condition of stddev > mean.
we have seen jobs where tasks have not been speculated for hours and this seems one explanation
why this may have happened. here's an example job with stddev > mean:
DataStatistics: count is 6, sum is 1.7141054797775723E-8, sumSquares is 2.9381575958035014E-16
mean is 2.8568424662959537E-9 std() is 6.388093955645905E-9
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